Usually, a brushless motor needs a detector for detecting the position of magnetic poles of a rotator of the motor. However, for example, in the case where it is difficult to use the pole position detector, there is employed a method in which the pole position detector is omitted and a commutation signal of the motor is generated on the basis of a voltage signal induced in an armature winding. This method will now be explained.
FIG. 1 is a diagram showing the construction of a brushless motor operating apparatus according to the above method. Reference numeral 1 designates a DC power source and numeral 2 designates a semiconductor switching element group which is composed of six transistors U to Z and six diodes connected in inverse parallel with the transistors. Numeral 3 designates a brushless motor which is composed of a three-phase connected armature winding 4 and a magnet rotator 5. Numeral 6 designates a filter circuit for waveform-processing a voltage signal induced in the armature winding 4, numeral 7 a comparison circuit for comparing output signals of the filter circuit 6, and numeral 8 a control circuit for making ON/OFF control of the semiconductor switching element group 2 in accordance with an output signal of the comparison circuit 7. The filter circuit 6 comprises a circuit in which an integrating circuit is connected behind a differentiating circuit, as shown in FIG. 4 (A) or a circuit in which a differentiating circuit is connected behind an integrating circuit, as shown in FIG. 4 (B).
An operation in the above-mentioned construction will be explained by use of FIGS. 2 and 3.
FIG. 2 shows a relationship between a voltage signal induced in the armature winding 4, an output signal of the filter circuit 6 and an output signal of the comparison circuit 7 for one phase or U phase. In the figure, an output signal obtained by waveform-processing an induced voltage V.sub.u for U phase by a filter circuit 6U is shown by 60U Also, induced voltages V.sub.v and V.sub.w for V and W phases are waveform-processed by filter circuits 6V and 6W so that they become 60V and 60W. A composite waveform of 60V and 60W is shown by 71U. The comparison circuit 7 compares the signals 60U and 71U to obtain a comparison circuit output signal 70U. This output signal serves as a position detection signal representative of the position of the magnet rotator 5.
The above-mentioned waveform processing is performed also for the V and W phases, thereby obtaining position detection signals 70V and 70W, respectively. The position detection signals 70V and 70W, respectively. The position detection signals 70U to 70W are signals which are different in phase from one another by 120.degree. C., as shown in FIG. 3. These position detection signals are subjected to a logical operation in the control circuit 8 to generate commutation signals 8U to 8Z. The transistors in the semiconductor switching element group 2 are switched by those commutation signals, thereby causing the brushless motor 3 to continuously generate a rotational torque. The above operation mode is called a position detection operation mode.
On the other hand, during a time when the brushless motor 3 is stopped, no induced voltage is generated. Upon starting, therefore, commutation signals 8U to 8Z as shown in FIG. 3 are externally applied at a low frequency to forcibly rotate the brushless motor 3 at a low speed. By this rotation, induced voltages are generated in the armature winding 4. The induced voltages are waveform-processed by the filter circuit 6 and are then compared in the comparison circuit 7 to obtain position detection signals 70U to 70W as shown in FIG. 3. The above operation mode is called a synchronized operation mode. At the point of time when such position detection signals have been settled in the synchronized operation mode, transition is made to a position detection operation mode in which the brushless motor is operated on the basis of the position detection signals.
However, the construction of the filter circuit as shown in FIG. 4 brings about the increase in size and cost of the apparatus since a high withstanding voltage on a level with the power source voltage is required for a condenser 6c of the differentiating circuit or a condenser 6e of the integrating circuit. Also, in many cases, an electrolytic condenser is used for the possession of a high withstanding voltage and an appropriate electrostatic capacitance and for reduction in size. However, since the electrolytic condenser has inferior high frequency characteristics, the stability of a position detection signal during a change, in load is deteriorated and hence the motor is liable to encounter an out-of-step phenomenon upon change in load.
Further, in both the circuit constructions shown in FIGS. 4(A) and 4(B), the condenser 6c in the differentiating circuit has no discharging path. Therefore, the condenser always has a DC component always when the brushless motor is stopped. When the brushless motor is started in such a state, the output of the filter circuit immediately after the start of a synchronized operation mode (for example, an output signal 60U for U phase) also has a DC component as shown by dotted line in FIG. 5. Accordingly, there is involved a problem that if the DC component is not attenuated upon transition to a position detection operation mode, a failure in transition is liable occur.